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Since the transformer usually costs less than the controller, you would want it to be the item that died under overload, assuming the controller itself didn't limit or cut the power. There used to be a special-order solenoid for Hydro-Rain valves that had an especially low current draw, that would be just perfect for your application. No longer manufactured.

By the way, what valve is giving 0.3 amp inrush spec? I would be assuming 0.4 to 0.5 when drawing up designs. If you do have a spare conductor, you can make it an extra common wire, and give yourself some wiggle room.

It seems to be an interesting question with no definite answer. I stopped at my supplier's today and got 3 different answers on it. The manager agreed with my interpredation, but there's no saying we're right...

So..the theoretical answer is as I described above. So.. a 1.5 amp output from the controller could fire a number of valves depending on their need... in my example above, I could do 5 valves. This is assuming no line loss. Digging around... 18 guage wire (which I am using) can go 1100 feet without appreciable loss. However, if there are two valves on it, we drop down to 680' or so. I can't find anything about adding another valve, but even extrapolating, I'd be running into a real short distance before line loss is an issue.

I've been warned if I try to draw too much amperage from the controller, I'd likely fry the transformer.

So... I'm going to try and keep it to no more than 2 valves/zone (controller documentation says it will support that) and save the 3/zone for real emergency issue resolution.

Looks like you're going to have to keep working on the math. Assuming you're using 18 guage solid copper, a quick google told me the following:
Current Capacity - 5 amps (you're good there)
Resistance - 0.75 Ohms/100ft.

So the real question is now not one of current (sounds like the wire and controller can handle that) but one of voltage. Will the voltage drop in the wire (V=IR) running multiple valves be small enough for the vavles to continute to operate propperly? Wet Boots' suggestion of doubling the common will reduce the resistance of the common wire, but what about the other side of the equation? Do you have one wire going to each valve, or will one wire run multiple valves?

Of course the fact that the resistance of the valves won't be the exact same for each valve AND the effective resistance changes (from open to close). So in the end, you might have to just rig a test to determine the answers to your question.

It would probably make more sense to consider reworking the zones so only 3 valves would run at a time. Is the mainline under constant pressure (are you using a master valve?) Extra valves will test your wiring's current-carrying ability, especially if you're using multiconductor cables. Double up conductors for the common.

Looking at things.... 1" valve needs .30 amp inrush to start (and .19 to hold)... controller outputs 1.5 amps. Would seem like the theoretical maximum would be 5 valves could open on a wire. Would probably stick to 4 to be safe if I could.

The entire situation is complex. Long story short: engineering drawings for the site had the wrong scale on them; site is actualy 70% bigger than planned. 900' of mainline is already run along with 20 zones worth of control wire buried. Nobody (especially me) wants to dig another wire in. But... with careful redesign, I can make the water end of things work if I can split zones around the property. Because of pavement & cement, etc it isn't practical to carry zones to their maximum hydraulic capacity, which is why I want to double/triple/quadruple up on using a wire and push multiple valves.

Since they don't manufacture controllers designed to run six valves on a zone, you might rethink your application. You could still use some mechanical controller, and however large a transformer you need.

Multi-valving a zone

OK...hardcore question []

I've got a situation where it looks as if we are going to need to run lots of valves simultaneously. Oddly enough, water volume isn't the issue (2" main) as much as distribution around the property (commercial site, tons of concrete, etc).

So.. how many valves can be run off of a wire? Oddly enough,I've never tried to run more than 2, but I think I've got a situation here where I may need to fire 4-6 valves as a zone..